Recently, there has been discussion about dynamic rate repartitioning (DRR) and seamless rate adaptation (SRA) and whether or not true seamlessness is really required or desirable. DRR refers to the capability to transfer data rate bandwidth between two bearer channels without changing the aggregate data rate. SRA refers to the capability to change the aggregate data rate.
DRR is often envisioned for the combined delivery of video and internet traffic on two latency paths. It is not desirable to put Internet traffic on an interleaved latency path because the delay slows the TCP/IP throughput and affects applications like gaming or video conferencing. Video traffic is very sensitive to impulse errors but not sensitive to delay and therefore using the interleaved path is appropriate. As video channels are switched on (off), bandwidth can be switched from (to) the fast latency path to (from) the interleaved latency path. For example, if there is more than one television, then it is desirable that switching one television on or off does not disturb the picture on another television. This is the primary motivation for a seamless DRR procedure.
Similarly with SRA, a subscriber might want to pay extra for a temporary increase in bandwidth. This subscriber would expect that the increase in charges would result in service that did not suffer delays and interruptions.
An interleaver/deinterleaver is a pair of building blocks normally used in a digital control and communication system to increase the stability of the system. In general, interleaving spreads the consecutive burst errors introduced into the system to many non-consecutive places so that errors may be easily detected or corrected by, for example, a forward error control (FEC) coding block. The interleaver and deinterleaver may be used together with Reed Solomon FEC code to combat the impulse noise on a twisted pair telephone line.
The current SRA and DRR in ADSL2 (Asychronous Digital Subscriber Line 2) is referred to as “seamless” but, when the interleaver is enabled, has a number of limitations and is not truly seamless. In ADSL2, an SRA or DRR is accomplished with an on-line reconfiguration (OLR) message. During an SRA or DRR, the bits and gains (bi, gi) can be changed as well as the number of bits in a DMT frame, Lp for latency path p. When the interleaver is enabled, the interleaver depth or codeword size can not be changed. Interleaver depth is defined as the quantity codewords stored in the interleaver's memory. Increasing the interleaver depth provides greater protection from bursts of noise in a DSL line. ADSL2 specifications are described in the International Telecommunications (ITU-T) G.992.3 Recommendation “Asymmetric Digital Subscriber Line (ADSL) transceivers 2,” which is incorporated by reference.
The delay through the interleaver and deinterleaver pair isinterleaver delay=8·(NFEC,p−1)(Dp−1)/(4·Lp) ms
where NFEC,p is the Reed-Solomon codeword size and Dp is the interleaver depth. The interleaver delay is inversely proportional to the number of bits carried in the latency path. For example, if the interleaver delay is 10 ms at a certain data rate with two video streams and a television is switched off, the interleaver delay would jump immediately to 20 ms if the data rate were cut in half. Depending on how the video buffering is done, this jump in latency may cause a momentary disruption in the picture and so it is not truly seamless.
Problems associated with an increase or decrease in the interleaver delay include:                an instant change in delay that could be registered as momentary disruption;        the period of the overhead channel in ADSL2 will fall out of the 15-20 ms range and violate the requirements of the recommendation;        a rate of the overhead channel changes in proportion to the change in data rate;        interleaver delay could exceed bounds required by the operator as the data rate is reduced;        for bonding, data rate must be changed on all bonded modems in exactly the same ratio to maintain the same buffering requirements and approximately and the same differential delay;        impulse noise protection, which can be written as 8*Dp*tp/Lp (DMT frames) where tp is the correction capability of the Reed-Solomon code, will change inversely proportional to the data rate change causing a possible change in the perceived picture quality; and        TCP/IP throughput performance will change        
True seamless rate change would mean no interruption in data and no change in the delay or perceived quality of the data stream. As the data rate changes, the way to achieve true seamless behavior when the interleaver is enabled is to change the interleaver depth in proportion to the data rate, as in equation (1), so that the overall delay remains constant. Because the codeword size does not change, the coding gain remains constant as does the immunity to impulse noise.
The problem is, when the interleaver depth changes, it is very difficult to structure things in a way that causes no interruption in data and no errors. Different vendors use different algorithms and memory structures to implement interleavers. All implementations, if done properly, result in the same output stream. But if the interleaver depth changes in the middle of the stream, it is very unlikely that two interleaver implementations will produce the same output during a transition period equal to the total end-to-end delay of the interleaver and deinterleaver. Even for a single implementation, the sequence of the output stream after a change in the interleaver depth can change depending on when the transition is made.
In the ADSL2 SRA and DRR schemes, the interleaver size remains constant meaning that the overall delay, impulse noise protection, overhead rate, and overhead period all change in proportion to the change in data rate. Thus, in a DSL system carrying voice, video, and or internet traffic, there is a desire among operators to be able to change the data rate or change the bandwidth allocation between high and low latency paths with minimal or no interruption in service. One solution (currently used in ADSL2) changes only the number of bits carried in each DMT frame but does not change the interleaver depth.